Cellulose ester film, production method of the same, film employed in liquid crystal display member, and polarizing plate

ABSTRACT

A film employed in a liquid crystal display member is disclosed. The film has film thickness of 20 to 60 μm and variation in the film thickness within ±3.0 percent of the standard film thickness.

FIELD OF THE INVENTION

The present invention relates particularly to a suitable film as thepolarizing plate protective film of a liquid crystal display unit.

A liquid crystal display unit (LCD) can be operated at low voltage aswell as low electric power consumption, can be directly connected to anIC circuit, and can be specifically produced in a shape having lessthickness. Accordingly, the liquid crystal display unit has been widelyemployed as a display unit for word processors, personal computers, andthe like.

Such an LCD is basically constructed in such a manner that, for example,both sides of a liquid crystal cell are provided with polarizing plates.

The polarizing plate transmits light having a polarization plain in thedefinite direction. Accordingly, the LCD plays an important role tovisualize the variation of orientation of liquid crystal due to anelectric field. Namely, the performance of the LCD largely depends onthat of the polarizing plate.

FIG. 1 shows a common structure of the polarizing plate. In FIG. 1,reference numeral 1 is a polarizer, and both sides of said polarizer 1are laminated with polarizing plate protective films 2. The LCD isconstructed by laminating a liquid crystal cell with the polarizingplate having such a structure.

Polarizer 1 is prepared in such a manner that iodine and the like areadsorbed onto a polymeric film and the resulting film is stretched.Specifically, after a solution called H ink containing a dichroicsubstance (iodine) is wet-adsorbed onto a polyvinyl alcohol film, thedichroic substance is allowed to orient in one direction by uniaxiallystretching the resulting film.

Employed as the polarizing plate protective film 2 are cellulose resins,especially cellulose triacetate.

Said polarizing plate protective film 2 is produced employingapparatuses of a drum casting system shown in FIG. 2 and a belt castingsystem shown in FIG. 3.

In the drum casting apparatus, casting die 12 is provided adjacent tocasting drum 11 and tenter drying section 14 via peeling roll 13, rolldrying section 15 and winding roll 16 are provided. After a dope fromthe casting die is cast onto the casting drum, the resulting film ispeeled from the casting drum 11, employing the peeling roll 13, is driedin a first, second, third, and fourth drying zones of the tenter dryingsection 14, completely dried in the roll drying section 15, and woundonto the winding roll 16.

In the belt casting apparatus shown in FIG. 3, casting die 22 isprovided adjacent to casting belt 21, and roll drying section 24 viapeeling roll 23 and winding roll 25 are provided. After a dope from thecasting die 22 is cast onto the casting belt 21, the resulting film ispeeled employing the peeling roll 23, completely dried in the rolldrying section 24, and then wound by the winding roll 25. In thiscasting belt, the cast dope is dried at a high rate. As a result, thetenter drying section employed in the drum casting system is notrequired.

At present, the thickness of the polarizing plate protective film 2 isat least 80 μm, which is relatively thick.

Recently, from the viewpoint of optical properties such as transmittanceand the like, a decrease in the thickness of the polarizing plateprotective film has been demanded.

However, when the polarizing plate protective film having a thickness ofbelow 80 μm is produced, it has been found that a number of wrinkles areformed in a film which is wound by the winding rolls 16 and 25 tomarkedly decrease the production yield.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a suitable filmemployed for a liquid crystal display member, which results in minimalwrinkles which are likely to be formed when said film has a thickness ofbelow 80 μm, increases the production yield, enhances transparency aswell as retardation due to a decrease in the thickness.

Furthermore, a decrease in the thickness of a visibility angle enlargingfilm provided in the entire surface of a liquid crystal displayapparatus and the like has been demanded, and said film has had the sameproblems as those of the polarizing plate protective film.

As the results of investigation performed by the present inventors, ithas been discovered that as variation in film thickness increases, morewrinkles are formed, while as variation of the same decreases, lesswrinkles are formed.

Further, as the results of detailed investigation on the ratio ofwrinkle formation as well as variation in film thickness, it has beendiscovered that at the film thickness of no more than 60 μm, whenvariation in film thickness is within ±3.0 percent of the standardthickness (average film thickness) of said film, is preferably ±2.9percent, and is more preferably ±2.8 percent, wrinkles, which are notpermitted as the polarizing plate protective film, are not formed.

Further, it has been discovered that when a film having a thickness ofno more than 60 μm is comprised of a cellulose ester film, frequency ofwrinkle formation largely depends on the degree of average substitution.The average degree of substitution as described herein is defined basedon the average number of hydroxyl groups among three of them in a unitcomposing cellulose, substituted with other substituents.

As the results of detailed investigation on the frequency of wrinkleformation as well as the average degree of substitution (the averagenumber in entire cellulose of hydroxyl groups of three of them in a unitof C₆H₁₀O₅ composing said cellulose substituted with othersubstituents), it has been discovered that at the film thickness of nomore than 60 μm, if the average degree of substitution is preferablybetween 2.88 and 3.00, is more preferably between 2.90 and 3.00, and ismost preferably between 2.91 and 3.00, wrinkles which are not permittedin the polarizing plate protective film, markedly decrease.

The present invention has been achieved based on the aforementioneddiscoveries, and the aforementioned problems are solved employing a filmfor a liquid crystal display unit, which has a film thickness of 20 to60 μm and a variation in film thickness within ±3.0 percent of thestandard film thickness. The variation in film thickness is preferablywithin ±2.9 percent of the standard film thickness, and is morepreferably within ±2.8 percent.

When the variation in film thickness is controlled within ±3.0 percentof the standard film thickness, in the case of a thin film in the rangeof 20 to 60 μm, especially in the range of 25 to 55 μm, and further inthe range of 30 to 55 μm, wrinkles are not substantially formed. Namely,wrinkles, which are not permitted in the film for a liquid crystaldisplay unit, especially a polarizing plate protective film, are notsubstantially formed. For example, when a 5000 m long film is wound,wrinkles, which are not permitted in the polarizing plate protectivefilm, are not formed. As a result, the production yield increases.

Further, when the film for a liquid crystal display unit is comprised ofcellulose ester film, adjusting the average degree of substitution tothe range of 2.88 to 3.00 effectively minimizes wrinkles.

Namely, when a film comprised of cellulose ester having an averagedegree of substitution of 2.88 to 3.00 is wound in a roll, the formationof wrinkles is markedly minimized as the film thickness decreases fromthe range of 20 to 60 μm via the range of 25 to 55 μm, to the range of30 to 55 μm. Namely, when the average degree of substitution ofcellulose ester is at least 2.88, is preferably at least 2.90, and ismore preferably at least 2.91, the film comprised of said celluloseester is not subjected to wrinkling even using the thin film having athickness of no more than 60 μm., that is, wrinkles which are notpermitted in the polarizing plate protective film are not substantiallyformed. For example, when a 5000 m long film was wound, wrinkles, whichwere not permitted in the polarizing plate protective film, were notfound. As a result, the high production yield was obtained.

The cellulose ester employed in the present invention preferably has acomposition ratio of wood pulp cellulose/cotton linter cellulose=60/40to 0/100 (in terms of weight ratio). When the polarizing plateprotective film is comprised of the cellulose ester having such a ratio,a high quality polarizing plate protective film, having minimalwrinkles, is obtained.

The polarizing plate film having a thickness of no more than 60 μm ispreferred because higher transparency as well as less phase differenceis obtained.

Further, the polarizing plate protective film having a thickness of nomore than 55 μm is more preferred. With the lower limit of thethickness, the thinner the film, the more it is preferred, if theresulting film exhibits sufficient mechanical strength. However, atpresent, from the viewpoint of the mechanical strength, the lower limitof the thickness is 20 μm, is preferably at least 25 μm, and is morepreferably at least 30 μm.

Namely, when conditions to achieve the thickness of 20 to 60 μm as wellas the variation in the film thickness within ±3.0 percent of thestandard film thickness are satisfied, and further, the condition toachieve the average degree of substitution of 2.00 to 3.00 aresimultaneously satisfied, first, problems with optical properties(transparency and phase difference properties) as well as wrinkle weresimultaneously overcome and a high quality polarizing plate protectivefilm was obtained.

The aforementioned film preferably has a tear strength of at least 7 g.The tear strength of at least 7 g is preferred, considering themechanical strength as the polarizing plate protective film, as well asthe winding operation during its production, which is carried at a highspeed while applying tension as high as possible to the film so thatwrinkles are not formed. The upper limit is not definite. However, whenthe element materials, which are now employed for the polarizing plateprotective film, are taken into account, the specific tear strength isno more than 50 g.

A film is preferably comprised particularly of lower fatty acid estersof cellulose. Further, the film preferably comprises plasticizers in anamount of 1 to 30 percent by weight. Still further, the film preferablycomprises an UV absorber in an amount of 0.01 to 5 percent by weight.

The film constructed, as described above, exhibits a haze of no morethan 0.5 percent (preferably no more than 0.4 percent). Further, thefilm has a retardation value of no more than 10 nm (preferably no morethan 8 nm, and more preferably no more than 4 nm). The film, having ahaze of no more than 0.5 percent, preferably no more than 0.4 percent aswell as a retardation value of no more than 10 nm, preferably no morethan 8, exhibits sufficient performance as the polarizing plateprotective film.

A film employed in a liquid crystal display member wherein filmthickness is between 20 and 60 μm and variation in the film thickness iswithin ±3.0 percent of the standard film thickness.

A film employed in a liquid crystal display member wherein said filmcomprises cellulose ester having a ratio of wood pulp cellulose/cottonlinter cellulose in the range of 40/60 to 0/100 (in weight ratio), andthe thickness of said film is between 20 and 60 μm and the variation inthe film thickness is within 3.0 percent of the standard film thickness.

A film employed in a liquid crystal display member wherein variation inthe thickness of said film, which is wound over a length of 1500 m, iswithin ±3.0 percent of the standard film thickness.

A film employed in a liquid crystal display member wherein the thicknessof said film is between 20 and 60 μm, and the variation in the thicknessof said film, which is wound over a length of 1500 m, is within ±3.0percent of the standard film thickness.

A cellulose ester film wherein a film thickness is 20 to 60 μm and anaverage degree of substitution is 2.00 to 3.00.

A cellulose ester film wherein a film thickness is 20 to 60 μm, and anaverage degree of substitution of the said film is 2.88 to 3.00, whensaid film having said thickness is wound in a roll over a length of atleast 1000 m after saponification.

A production method of a cellulose ester film wherein a dope compositionprepared by dissolving in a solvent composition materials for said film,which comprises cellulose ester having an average degree of substitutionof 2.00 to 3.00, is cast into a film having a thickness of 20 to 60 μmemploying a solution casting method.

A polarizing plate wherein said plate is provided with said celluloseester film and a polarizer in which said cellulose ester film is overlayon said polarizer.

A polarizing plate wherein said plate is provided with said celluloseester film and a polarizer in which said cellulose ester film is overlayon both surfaces of said polarizer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a polarizing plate.

FIG. 2 is a schematic view of the casting apparatus of a polarizingplate protective film.

FIG. 3 is a schematic view of the casting apparatus of a polarizingplate protective film.

FIG. 4 is a schematic view of a liquid crystal display unit.

DETAILED DESCRIPTION OF THE INVENTION

The thickness of the film for a liquid crystal display unit ispreferably between 20 and 60 μm, and a variation in the film thicknessis preferably within ±3.0 percent of the standard film thickness.

The film thickness is more preferably between 25 and 55 μm, and is mostpreferably between 30 and 55 μm. The variation in film thickness is morepreferably within ±2.9 percent of the standard film thickness, and ismost preferably within ±2.8 percent.

The variation in film thickness is measured as follows. When thevariation in film thickness is measured during production or for a longfilm having a length of at least 1000 m or at least 1500 m, the filmthickness is measured at every 50 mm in the width direction as well asin the casting direction, employing a β-ray film thickness meter.Standard film thickness is obtained by averaging all measurements.Further, variation in film thickness is obtained by expressing themaximum deviation from the standard film thickness in percentage.Furthermore, when the variation in film thickness in a film having animage size provided with a liquid crystal display unit, the filmthickness is measured at every 50 mm in the long direction as well as inthe short direction and the standard film thickness is obtained byaveraging all measurements. Further, the variation in film thickness isobtained by expressing the maximum deviation from the standard filmthickness in percentage.

Furthermore, the film for the liquid crystal display unit of the presentinvention may be comprised of a single layer or a multilayer. The filmfor the liquid crystal display unit include a polarizer protective filmfor protecting a polarizer, a visibility angle increasing film which isprovided in front of the liquid crystal display unit and increases avisibility angle, a glare minimizing film, a non-reflective film, andthe like. However, the present invention is not limited to these.Specifically, the film of the present invention is preferably employedas the polarizing plate protective film.

Employed as films for the liquid crystal display unit may be varioustypes of transparent films. For example, are cited cellulose esterfilms, polycarbonate based films, polyacrylate based films, norbornanebased resin films, syndiotactic polystyrene based films, polysulfonebased films, polyester based resin and the like. Of these, the celluloseester films are preferred. Furthermore, the transparent films asdescribed herein preferably transmit at least 50 percent of visiblelight, more preferably transmits at least 70 percent, and mostpreferably transmit at least 80 percent.

The transparent support preferably has dimensional shrinkage ratio afterstored for 3 hours at 80° C. of 0.05% or less, more preferably 0.03% orless.

The transparent support preferably has coefficient water absorption of1.0 to 4.5%, more preferably 1.0 to 3.0%, particularly 1.0 to 2.5%.

A cellulose ester film, which is employed as the film for the liquidcrystal display unit, preferably has an average degree of substitutionof 2.88 to 3.00. The average degree of substitution is more preferablybetween 2.90 and 3.00, and is most preferably between 2.91 and 3.00.

Further, when after saponification, a film is wound in a roll over alength of at least 1000 m, the effects of the present invention aremarkedly exhibited. When the length of film is at least 1500 m, theeffects of the present invention are more markedly exhibited, and whenthe length of film is at least 3000 m, the effects of the presentinvention are further more markedly exhibited. When the length of filmis at least 4000 m, the effects of the present invention are stillfurther more markedly exhibited.

Furthermore, a cellulose ester film, which is employed as the film forthe liquid crystal display unit, is preferably comprised of the lowerfatty acid ester of cellulose in which hydroxyl groups of said celluloseare substituted with lower fatty acids. Specifically, the average degreeof substitution of three hydroxyl groups in cellulose, substituted withlower fatty acids, is preferably between 2.88 and 3.00.

Herein, the lower fatty acids in the lower fatty acid esters ofcellulose mean fatty acids having no more than 6 carbon atoms, andinclude, for example, cellulose acetates such as cellulose diacetate,cellulose triacetate, and the like, and fatty acid ester mixtures suchas cellulose propionate, cellulose butyrate, cellulose acetatepropionate, cellulose acetate butyrate and the like. The most preferredlower fatty acid ester is cellulose triacetate. Particularly cellulosetriacetate having an acetated degree of 54 to 62 percent (mostpreferably at least 59 percent) is preferred. Further, cellulosetriacetate having a degree of polymerization of 250 to 400 is preferred.

Further, a film employing cellulose ester having a ratio of wood pulpcellulose/cotton linter cellulose in the range of 60/40 to 0/100 interms of weight ratio is preferred.

Sum of content of mannose and xylose in the film is preferably 6 mol %or less, more preferably 4 mol % or less, in order to make easyseparating the film in manufacturing process.

Further, the tear strength of the film for the liquid crystal displayunit is preferably at least 7 g. The tear strength can be measured by amethod shown in ISO 6383/2-1983. Still further, the haze of the same ispreferably no more than 0.5 percent. Further, the retardation value ofthe same is preferably below 10 nm. The retardation value can bemeasured employing an automatic double refractometer. For instance,KOBRA 21DH (manufactured by KS Systems Co., Ltd.) and the like arecited.

If desired, in addition to the cellulose ester, the aforementioned filmcomprises matting agents such as fine silicon dioxide particles and thelike.

Furthermore, said film preferably comprises one type or at least twotypes of UV absorbers. From the viewpoint of minimizing of thedegradation of liquid crystal, UV absorbers are preferred, which exhibitefficient absorption for ultraviolet radiation having a wavelength of nolonger than 370 nm and exhibit minimal absorption for visible lighthaving a wavelength of no shorter than 400 nm. For instance, are listedoxybenzophenone based compounds, benzotriazole based compounds,salicylic acid ester based compounds, benzophenone based compounds,cyano acrylate based compounds, nickel complex based compounds, and thelike. Particularly preferred UV absorbers are benzotriazole basedcompounds as well as benzophenone based compounds. Of these,benzotriazole compounds are preferred because they result in lesscoloration, which is not required for the cellulose ester. The addedamount of UV absorbers is between 0.01 and 5 percent by weight(specifically between at least 0.5 and no more than 3 percent byweight).

Further, one type or at least two types of plasticizers are preferablyincorporated. Preferably are incorporated, for example, phosphoric acidester based plasticizers such as triphenyl phosphate, tricresylphosphate, cresyldiphenyl phosphate, octyldiphenyl phosphate,diphenylbiphenyl phosphate, trioctyl phosphate, tributyl phosphate, andthe like, and glycolic acid ester based plasticizers such as triacetin,tributyrin, butylphthalylbutyl glycolate, ethylphthalylethyl glycolate,methylphthalylethyl glycolate, butylphthalylbutyl glycblate, and thelike. Of these, phthalic acid ester based and glycolic acid ester basedplasticizers are preferred because these are not likely to result inhydrolysis of the cellulose ester. Furthermore, plasticizers having asolidifying point (an intrinsic solidifying point described inENCYCLOPEDIA CHIMICA published by Kyoritsu Shuppan Co.) of no more than20° C. are preferably incorporated. Such plasticizers include, forexample, tricresyl phosphate, cresylphenyl phosphate, tributylphosphate, diethyl phthalate, dimethyl phthalate, dioctyl phthalate,dibutyl phthalate, di-2-ethylhexyl phthalate, triacetin,ethylphthalylethyl glycolate, and the like. The content of plasticizersis between 1 and 30 percent by weight from the viewpoint of dimensionalstability (specifically between at least 2 and no more than 20 percentby weight, and more specifically at least 10 and no more than 15 percentby weight). Further, the content of plasticizers having a solidifyingpoint of no more than 20° C., especially no more than 14° C. ispreferably increased because the flexibility of the cellulose ester filmas well as machining properties of the film (such as slitting andstamping; when such machining is not well carried out, a cutting planeresults in a saw-tooth shape and chips are generated, and when suchchips are adhered onto the film, the performance of a liquid crystaldisplay is deteriorated) is improved. Based on these, all plasticizersmay have a solidifying point of no more than 20° C., especially no morethan 14° C.

Solvents to compose a dope composition include, for example, alcohols(particularly, lower alcohols) such as methanol, ethanol, n-propylalcohol, iso-propyl alcohol, n-butanol, and the like, and aliphatichydrocarbons and chlorides thereof such as cyclohexane, dioxane,methylene chloride and the like. Regarding the content ratio ofsolvents, for example, the content ratio of methylene chloride ispreferably between 70 and 95 percent by weight, and that of othersolvents is preferably between about 5 and about 30 percent by weight.

Heating temperature after the addition of solvents is preferably betweenno less than the boiling point of employed solvents and no more than theboiling point of the resulting solution. For example, the temperature isset at no less than 60° C., particularly between 80 and 110° C. Pressureis applied so that the solution does not boil at the set temperature.

After dissolution, the resulting solution may be removed from the vesselduring cooling, or may be removed from the vessel employing a pump orthe like, cooled employing a heat exchanger, and employed afterfiltration.

A dope composition is prepared by dissolving the aforementioned filmcomposing materials (such as cellulose ester, plasticizers, UVabsorbers, and the like) in solvents, and the resulting dope is thencast employing a solution casting method. Thus the film of the presentinvention is obtained, which employs the lower fatty acid ester ofcellulose as the raw material, and has a thickness of 20 to 60 μm(specifically, at least 25 μm, further, at least 30 μm, and no more than55 μm), has a variation in the film thickness within ±3.0 percent(preferably within ±2.9 percent and more preferably within ±2.8percent), and has an average degree of substitution of 2.88 to 3.00(preferably at least 2.90 and more preferably at least 2.91).Specifically, the film according to the present invention is obtainedwhich has a tear strength of at least 7 g, a haze of no more than 0.5percent (particularly no more than 0.4 percent) and a retardation of nomore than 10 nm (particularly no more than 8 nm).

In the case of the production of said film, techniques are utilizedwhich are described in, for example, U.S. Pat. Nos. 2,492,978,2,789,070, 2,739,069, 2,492,977, 2,336,310, 2,367,603, and 2,60,7704;British Patent Nos. 64,071 and 735,892; and Japanese Patent PublicationNos. 45-9074, 49-4554, 49-5614, 60-27562, 61-39890, and 62-4208.

The film according to the present invention is obtained by regulatingthe amount of solvents, a gap between casting dies, a casting speed, anda winding tension at the high residual volume of solvents as well as atthe low residual volume of solvents. Specific values of theses varydepending on the dope composition and other conditions. Specifically, avariable for each of the aforementioned conditions is optionally set andat the time, said variable is regulated so that the variation in filmthickness is in the range of the present invention.

Furthermore, the film of the present invention is more preferablyproduced employing a belt casting system apparatus as shown in FIG. 3,compared to a drum casting system apparatus as shown in FIG. 2 Thetechniques described in the following references may be applied;

U.S. Pat. Nos. 2,492,978, 2,739,070, 2,739,069, 2,492,977, 2,336,310,2,367,603 and 2,607,704, British Patent Nos. 64,071 and 735,892, JP-BNos. 45-9074, 49-4554, 49-5614, 60-27562, 61-39890 and 62-4208.

The film satisfying the requirement of the present invention may beobtained by adjusting solvent content, gap of extrusion die, castingspeed, and winding tensions at the time of high residual solvent contentand at the time of low residual solvent content. The practical valuesvary depending upon dope composition or other condition, and therefore,parameters of the values which are provided adequately, should beadjusted so that the variation of film thickness satisfies therequirement of the present invention.

It is preferable to employ an apparatus casting on belt shown by FIG. 3rather than an apparatus casting on drum shown by FIG. 2, when the filmis prepared.

The liquid crystal display member of the present invention, for example,the polarizing plate is produced as follows.

For instance, the polarizing plate protective film, according to thepresent invention, for example, the alkali-treated cellulose ester filmaccording to the present invention is adhered employing an aqueoussolution of perfectly saponified polyvinyl alcohol on at least one sideor preferably both sides of a polarizer prepared by adsorbing iodine andthe like to a polymeric film followed by stretching. Instead of thealkali treatment, there may be employed methods described in JapanesePatent Publication Open to Public Inspection Nos. 6-94915 as well as6-118232.

As described above, a polarizing plate comprised of polarizer 1 which issandwiched with two sheets of polarizing plate protective film 2 isproduced. Said polarizing plate comprises a polarizer as well as aprotective film. The thickness of said protective film is between 20 and60 μm, and the variation in the film thickness of said protective filmis within ±3.0 percent of the standard film thickness. When saidprotective film is cellulose ester film, the average degree ofsubstitution is preferably between 2.88 and 3.00.

Polarizer is preferably prepared with polyvinylalcohol orethylene-vinylalcohol when the film of the invention is applied to thepolarizing plate protect film. Polarizer prepared by blendingethylene-vinyl alcohol copolymer and polyvinyl alcohol is particularlypreferable. In this case, content of ethylene-vinyl alcohol copolymerwith reference to 100 parts of polyvinyl alcohol is preferably 0.1 to1000 weight parts, more preferably 0.5 to 200 parts by weight andparticularly preferably 1 to 100 parts by weight.

Ethylene content in ethylene-vinyl alcohol is preferably 1 to 25 mol %,more preferably 2 to 10 mol %, particularly preferably 5 to 10 mol %.Polymerization degree of the ethylene-vinyl copolymer is preferably 500or more.

The blend of ethylene-vinyl alcohol copolymer and polyvinyl alcohol ispreferably uniaxially stretched to prepare film.

Durability of the polarizing plate may be improved by employing thepolarizer. The polarizer is subjected to immersing into solution ofboric acid, which is crosslinking agent, for the purpose of improvingdurability of polarizing plate.

The liquid crystal display members as described in the presentspecification are members employed in a liquid crystal display unit,which include, for example, in addition to the aforementioned polarizingplate, a polarizing plate protective film, a phase difference plate, areflective plate, a visibility angle increasing film, a glare minimizingfilm, a non-reflective film, an antistatic film, and the like.

Of these, the film of the present invention is suitable for thepolarizing plate protective film.

One example of the crystal display unit of the present invention will bebriefly described with reference to a schematic view in FIG. 4. Saidcrystal display unit comprises first polarizing plate 31, liquid crystalcell 4, and second polarizing plate 32 provided in the interior of saidfirst polarizing plate 31 as well as said liquid crystal cell 4.Furthermore, as a light source, back light 5 may be provided in thefurther interior of said second polarizing plate 32. Further, the firstpolarizing plate 31 is comprised of first polarizer 110, firstprotective film 21 provided on the surface of the first polarizer 110,which does not face the liquid crystal cell 4, and protective film 22provided on the surface-of the first polarizer 110, which faces theliquid crystal cell 4. Further, the second polarizing plate 32 iscomprised of second polarizer 120, third protective film 23 provided onthe surface of the second polarizer 120, which faces the liquid crystalcell 4, and fourth protective film 24 provided on the surface of thepolarizer 32, which does not face the liquid crystal cell 4.Furthermore, at least one of the first protective film, the secondprotective film, the third protective film, and the fourth protectivefilm has a film thickness of 20 to 60 μm as well as a variation in filmthickness within ±3.0 percent. The particular preferable way is thateach-of the second protective film and the third protective film has afilm thickness of 20 to 60 μm as well as a variation in film thicknesswithin ±3.0 percent. In this instance, the first protective film and thefourth protective film may be made thicker. However, the protective filmaccording to the present invention is preferably employed for the fourthprotective film. Particular preferably the protective film according tothe present invention is preferably employed for the first protectivefilm.

Further, when the protective film, having a film thickness of 20 to 60μm, is comprised of a cellulose ester film, the average degree ofsubstitution of said cellulose ester film is preferably between 2.88 and3.00. Of course, employed as these protective films may be theaforementioned film for the liquid crystal display unit.

EXAMPLE 1

Dope Composition Triacetyl cellulose having an acetated degree of 61.0100 weight parts percent, employing the cellulose of wood pulpcellulose/cotton linter cellulose = 50/50 Plasticizer having thefollowing formula  4 weight parts

Tinuvin 326 (UV absorber manufactured by Ciba  0.5 weight part SpecialtyChemicals Co.) Tinuvin 328 (TN absorber manufactured by Ciba  0.5 weightpart Specialty Chemicals Co.) Methylene chloride 250 weight artsMethanol 250 weight parts

The aforementioned composition was placed in a tight sealed vessel,heated while stirring at 70° C. under applied pressure, and perfectlydissolved. Thereafter, the resulting dope was filtrated.

The filtrated dope was then cast employing a solution casting method.Namely, a belt casting apparatus shown in FIG. 3 was employed, and saiddope having a temperature of 33° C. was uniformly cast onto a stainlesssteel belt support. While regulating the temperature of the stainlesssteel belt support at 25° C., solvents were evaporated, and theresulting film was peeled from the stainless steel belt support.Thereafter, the film was conveyed employing many rolls in a drying zoneand dried. The resulting cellulose triacetate film having a width of1300 mm as well as a standard film thickness of 40 μm was wound under awinding tension of 10 kg/m over a length of 3000 m. Further, afterpeeling, a film portion having a residual solvent content of 25 percentby weight was conveyed under a winding tension of 14 kg/m (width), whichwas gradually reduced during drying, and was wound under a windingtension of 10 kg/m (width). Further, the residual solvent content wascontrolled so as to obtain 3 percent by weight.

Variation in film thickness of said cellulose triacetate film over alength of 3000 m was ±0.4 μm (±1.0 percent).

Further, said cellulose triacetate film exhibited a haze of 0.3 percent,a retardation value (Re) of 2 nm, and a tear strength of 8 g.

An alkali treated film was obtained in such a manner the said cellulosetriacetate was treated with a 2.5N aqueous sodium hydroxide solution at40° C. for 60 seconds, washed for 3 minutes, and was subjected toformation of a saponified layer.

A polarizer was prepared by immersing a 120 μm thick polyvinyl alcoholfilm into 100 weight parts of an aqueous solution containing one weightpart of iodine and 4 weight parts of boric acid and stretching fourtimes the resulting film.

Further, a polarizing plate was prepared by adhering the aforementionedalkali-treated cellulose triacetate film onto both surfaces of theresulting polarizer, employing a 5% aqueous perfectly saponifiedpolyvinyl alcohol solution as the adhesive.

EXAMPLE 2

A film was prepared by repeating the procedure of Example 1 with theexemption that in casting conditions of Example 1, the winding tensionfor the portion, having a residual solvent amount of 25 weight percent,was replaced with 18 kg/m (width), the winding tension, when the film iswound onto a winding core, was replaced with 9 kg/m (width), and theresidual solvent amount, when wound onto the winding core, was replacedwith 5 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 40 μm, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of ±0.7 μm (±1.0 percent), a haze of 0.3 percent, aretardation value (Re) of 2 nm, and a tear strength of 8 g.

EXAMPLE 3

A film was prepared by repeating the procedure of Example 1 with theexception that in casting conditions of Example 1, the winding tensionfor the portion, having a residual solvent amount of 25 weight percent,was replaced with 10 kg/m (width), and the residual solvent amount, whenwound onto a winding core, was replaced with 5 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 40 μm, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of 1.0 μm (±2.5 percent), a haze of 0.3 percent, aretardation value (Re) of 2 nm, and a tear strength of 8 g.

EXAMPLE 4

A film was prepared by repeating the procedure of Example 1 with theexception that in casting conditions of Example 1, the winding tensionfor the portion, having a residual solvent amount of 25 weight percent,was replaced with 25 kg/m (width), the winding tension, when the film iswound onto a winding core, was replaced with 12 kg/m (width), and theresidual solvent amount, when the film was wound onto the winding core,was replaced with 5 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 40 μm, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of ±1.2 μm (±3.0 percent), a haze of 0.3 percent, aretardation value (Re) of 2 nm, and a tear strength of 8 g.

EXAMPLE 5

A film was prepared by repeating the procedure of Example 1 with theexception that a dope component, triacetyl cellulose (cellulose having aratio of wood pulp cellulose/cotton liter cellulose=50/50, was employed,and the acetated degree was 61.0 percent), which was employed in Example1, was replaced with triacetyl cellulose (cellulose having a ratio ofwood pulp cellulose/cotton liter cellulose=70/30, was employed, and theacetated degree was 61.0 percent), and in the casting conditions, thewinding tension for the portion, having a residual solvent amount of 25weight percent, was replaced with 18 kg/m (width), the winding tension,when the film is wound onto a winding core, was replaced with 9 kg/m(width), and the residual solvent amount, when the film was wound ontothe winding core, was replaced with 3 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 40 μm, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of ±1.0 μm (±2.5 percent), a haze of 0.3 percent, aretardation value (Re) of 2 nm, and a tear strength of 8 g.

EXAMPLE 6

A film was prepared by repeating the procedure of Example 1 with theexception that in casting conditions of Example 1, the winding tensionfor the portion, having a residual solvent amount of 25 weight percent,was replaced with 19 kg/m (width), the winding tension, when the film iswound onto a winding core, was replaced with 9 kg/m (width), and theresidual solvent amount, when the film was wound onto the winding core,was replaced with 2 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 50 μm, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of ±1.0 μm (±2.0 percent), a haze of 0.35 percent, aretardation value (Re) of 3 nm, and a tear strength of 10 g.

EXAMPLE 7

A film was prepared by repeating the procedure of Example 1 with theexception that in casting conditions of Example 1, the winding tensionfor the portion, having a residual solvent amount of 25 weight percent,was replaced with 15 kg/m (width), the winding tension, when the film iswound onto a winding core, was replaced with 9 kg/m (width), and theresidual solvent amount, when wound onto the winding core, was replacedwith 3 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 35 m, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of ±0.7 μm (±2.0 percent), a haze of 0.25 percent, aretardation value (Re) of 1 nm, and a tear strength of 7 g.

COMPARATIVE EXAMPLE 1

A film was prepared by repeating the procedure of Example 1 with theexception that in casting conditions of Example 1, the winding tensionfor the portion, having a residual solvent amount of 25 weight percent,was replaced with 10 kg/m (width), the winding tension, when the film iswound onto a winding core, was replaced with 10 kg/m (width), and theresidual solvent amount, when the film was wound onto the winding core,was replaced with 5 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 40 m, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of ±2.0 μm (±5.0 percent), a haze of 0.3 percent, aretardation value (Re) of 2 nm, and a tear strength of 8 g.

COMPARATIVE EXAMPLE 2

A film was prepared by repeating the procedure of Example 1 with theexception that in casting conditions of Example 1, the winding tensionfor the portion, having a residual solvent amount of 40 weight percent,was replaced with 15 kg/m (width), the winding tension, when the film iswound onto a winding core, was replaced with 15 kg/m (width), and theresidual solvent amount, when the film was wound onto the winding core,was replaced with 5 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 40 m, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of ±1.5 μm (±3.8 percent), a haze of 0.3 percent, aretardation value (Re) of 3 nm, and a tear strength of 8 g.

COMPARATIVE EXAMPLE 3

A film was prepared by repeating the procedure of Example 1, except thata dope component, triacetyl cellulose (employing cellulose having aratio of wood pulp cellulose/cotton liter cellulose=50/50, with anacetated degree of 61.0 percent) which was employed in Example 1 wasreplaced with triacetyl cellulose (employing cellulose having a ratio ofwood pulp cellulose/cotton liter cellulose=70/30, with an acetateddegree of 61.0 percent), and in the casting conditions, the windingtension for the portion, having a residual solvent amount of 25 weightpercent, was replaced with 10 kg/m (width), the winding tension, whenthe film was wound onto a winding core, was replaced with 10 kg/m(width), and the residual solvent amount when the film was wound ontothe winding core, was replaced with 5 percent by weight.

The resulting cellulose triacetate film having a width of 1300 mm aswell as a standard film thickness of 50 m, which was wound under awinding tension of 10 kg/m over a length of 3000 m, had a variation infilm thickness of ±1.8 μm (±3.6 percent), a haze of 0.3 percent, aretardation value (Re) of 3 nm, and a tear strength of 10 g.

Properties

Degree of wrinkling was visually evaluated for each of wound filmsprepared in the aforementioned examples. Table 1 shows the results.TABLE 1 Variation Degree of Degree of Film in Film Haze Re TearWrinkling Wrinkling Thickness Thickness (%) (nm) Strength (1) (2)Example 1 40μ ±1.0% 0.3 2 8 g A A A A Example 2 40 ±1.8% 0.3 2 8 g A A AA Example 3 40 ±2.5% 0.3 2 8 g A A A A Example 4 40 ±3.0% 0.3 3 8 g A AA A Example 5 40 ±2.5% 0.3 2 8 g A B A B Example 6 50 ±2.0% 0.35 3 10 g A A A A Example 7 35 ±2.0% 0.25 1 7 g A A A A Comparative 40 ±5.0% 0.3 28 g B C C C Example 1 Comparative 40 ±3.8% 0.3 3 8 g B B C C Example 2Comparative 50 ±3.6% 0.3 3 10 g  B B C C Example 3* Degree of Wrinkling (1) was obtained by inspecting wrinkles formed inthe portion which was wound over a length of 1500 m, while Degree ofWrinkling (2) was obtained by inspecting wrinkles formed in the portionwhich was wound over a length of 3000 m. FIG. 30 indicates a windingspeed of 30 m/minutes, while 50 indicates a winding speed of 50m/minutes.“A” indicates that no wrinkles were observed;“B” indicates that the formation of minimal wrinkles was observed; and“C” indicates that the formation of wrinkles was definitely observed.

EXAMPLE 8

Dope Composition

Triacetyl cellulose having an

-   -   average degree of substitution of 2.95, employing cellulose of        wood pulp cellulose/cotton linter cellulose=26/75 100 weight        parts

Triphenyl phosphate 2 weight parts

Ethylphthalyl glycolate 3 weight parts

Tinuvin 326 (UV absorber manufactured by Ciba Specialty Chemicals Co.)0.5 weight part

Tinuvin 328 (UV absorber manufactured by Ciba Specialty Chemicals Co.)0.5 weight part

Methylene chloride 250 weight arts

Methanol 250 weight parts

The aforementioned composition was placed in a tight sealed vessel,heated while stirring at 70° C. under applied pressure, and perfectlydissolved. The resulting dope was then filtrated.

Thereafter, the filtrated dope was cast employing a solution castingmethod. Namely, a belt casting apparatus shown in FIG. 3 was employed,and said dope at 33° C. was uniformly cast onto a stainless steel beltsupport. While maintaining the temperature of the stainless steel beltsupport at 25° C., solvents were evaporated, and the resulting film waspeeled from the stainless steel belt support. The film was then conveyedemploying many tolls in a drying zone and dried. The resulting cellulosetriacetate film having a width of 1300 mm as well as a standard filmthickness of 30 μm was wound under a winding tension of 10 kg/m (width)over a length of 3000 m. Further, after peeling, the film portion,having a residual solvent content of 25 percent by weight, was conveyedunder a winding tension of 15 kg/m (width), which was gradually reducedduring drying, and was wound under a winding tension of 8 kg/m (width).Further, the residual solvent content was controlled so as to obtain 3percent by weight.

Variation in film thickness of said cellulose triacetate film, having astandard film thickness (average film thickness) of 30 μm over a lengthof 3000 m, was ±0.6 μm (±2.0 percent). The film thickness was a valueobtained by employing an automatic film thickness on-line measuringapparatus manufactured by Yokokawa Denki Co.

Further, said cellulose triacetate film had a haze of 0.35 percent, aretardation value (Re) of 3 nm, and a tear strength of 10 g.

An alkali treated film was obtained in such a manner the said cellulosetriacetate specified as above was treated with a 2.5N aqueous sodiumhydroxide solution for 60 seconds, washed for 3 minutes, and wassubjected to formation of a saponified layer.

Furthermore, a polarizer was prepared by immersing a 120 μm thickpolyvinyl alcohol film into 100 weight parts of an aqueous solutioncontaining one weight part of iodine and 4 weight parts of boric acid,and stretching four times the resulting film.

Further, a polarizing plate was prepared by adhering the aforementionedalkali-treated cellulose triacetate film on both surfaces of theresulting polarizer, employing a 5% aqueous perfectly saponifiedpolyvinyl alcohol solution as the adhesive.

EXAMPLE 9

A film was prepared by repeating the procedure of Example 8 with theexception that in casting conditions of Example 8, 15 kg (width) of thewinding tension for the portion, having a residual solvent amount of 25weight percent, was replaced with 14 kg/m (width), and 3 percent byweight of the residual solvent amount, when the film was wound onto thewinding core, was replaced with 5 percent by weight.

The cellulose triacetate film having a width of 1300 mm as well as astandard film thickness of 30 μm, prepared as described above, had avariation in film thickness of ±0.7 μm (±2.3 percent), a haze of 0.25percent, a retardation value (Re) of 1 nm, and a tear strength of 7 g.

EXAMPLE 10

A film was prepared by repeating the procedure of Example 9 with theexception that the triacetyl cellulose having an average degree ofsubstitution of 2.95, which was employed in Example 9, was replaced withone prepared by saponifying said triacetyl cellulose (having an averagedegree of substitution of 2.95 which was not substantially varied due tothe saponification).

The cellulose triacetate film having a width of 1300 mm as well as astandard film thickness of 30 μm, prepared as described above, had avariation in film thickness of ±0.7 μm (+2.3 percent), a haze of 0.24percent, a retardation value (Re) of 1 nm, and a tear strength of 7 g.

EXAMPLE 11

A film was prepared by repeating the same procedure with the exceptionthat triacetyl cellulose having an average degree of substitution of2.95, employed in Example 8, was replaced with triacetate film having anaverage degree of 2.93, and in casting conditions of Example 1, 15 kg/m(width) of the winding tension for the portion, having a residualsolvent amount of 25 weight percent, was replaced with 18 kg/m (width),8 kg/m (width) of the winding tension, when the film is wound onto awinding core, was replaced with 10 kg/m (width), and further, 3 percentby weight of the residual solvent amount, when the film was wound ontothe winding core, was replaced with 5 percent by weight.

The cellulose triacetate film having a width of 1300 mm as well as astandard film thickness of 40 μm, prepared as described above, had avariation in film thickness of ±1.2 μm (±3.0 percent), a haze of 0.30percent, a retardation value (Re) of 3 nm, and a tear strength of 8 g.

EXAMPLE 12

A film was prepared by repeating the procedure of Example 11 with theexception that in casting conditions of Example 11, 18 kg/n of thewinding tension for the portion, having a residual solvent amount of 25weight percent, was replaced with 25 kg/m (width), and 10 kg/m (width)of the winding tension, when the film is wound onto a winding core, wasreplaced with 12 kg/m (width).

The cellulose triacetate film having a width of 1300 mm as well as astandard film thickness of 40 μm, prepared as described above, had avariation in film thickness of ±0.4 μm (+1.0 percent), a haze of 0.30percent, a retardation value (Re) of 2 nm, and a tear strength of 8 g.

EXAMPLE 13

A film was prepared by repeating the procedure with the exception thattriacetyl cellulose, having an average degree of substitution of 2.95,employed in Example 8, was replaced with triacetate film having anaverage degree of 2.90, and in casting conditions of Example 1, 15 kg/m(width) of the winding tension for the portion, having a residualsolvent amount of 25 weight percent, was replaced with 20 kg/m (width),8 kg/in (width) of the winding tension, when the film is wound onto awinding core, was replaced with 10 kg/m (width), and further, 3 percentby weight of the residual solvent amount, when the film was wound ontothe winding core, was replaced with 5 percent by weight.

The cellulose triacetate film having a width of 1300 mm as well as astandard film thickness of 40 μm, prepared as described above, had avariation in film thickness of ±1.0 μm (±2.5 percent), a haze of 0.30percent, a retardation value (Re) of 2 nm, and a tear strength of 8 g.

EXAMPLE 14

A film was prepared by repeating the procedure with the exception thatin casting conditions of Example 13, 20 kg/m (width) of the windingtension for the portion, having a residual solvent amount of 25 weightpercent, was replaced with 18 kg/m (width), and 10 kg/m (width)of thewinding tension, when the film is wound onto a winding core, wasreplaced with 8 kg/m (width).

The cellulose triacetate film having a width of 1300 mm as well as astandard film thickness of 40 μm, prepared as described above, had avariation in film thickness of ±0.7 μm (±1.75 percent); a haze of 0.30percent, a retardation value (Re) of 2 nm, and a tear strength of 8 g.

(Properties)

Degree of wrinkling was visually evaluated for each of wound filmsprepared in the aforementioned Examples. Table 2 shows the results.TABLE 2 Degree of Wrinkling Degree of Wrinkling during Winding 1500 mduring Winding 3000 m Winding Speed Winding Speed 30 m/min 50 m/min 30m/min 50 m/min Example 8 A A A A Example 9 A A A A Example 10 A A A AExample 11 A A A B Example 12 A A A A Example 13 A A A C Example 14 A AA BA: no wrinkles were observedAB: wrinkles were slightly observedB: some wrinkles were observedC: wrinkles were clearly observed

Namely, wrinkles of the cellulose ester film according to the presentinvention, when wound in a roll shape, are minimized to increase theyield, and the decrease in film thickness enhances the transparency aswell as the retardation. Accordingly it is found that said film issuitably employed as a liquid crystal display member.

Wrinkles are minimized which tend to occur, when the thickness of a filmdecreases, and the yield increases. The decrease in film thicknessenhances the transparency as well as the retardation. As a result, asuitable film for a liquid crystal display member is obtained.

Accordingly, a polarizing plate may be obtained which exhibits hightransparency, minimal degradation, and stabilized quality. Further,liquid crystal display units having excellent visibility may beconsistently obtained.

Disclosed embodiment can be varied by a skilled person without departingfrom the spirit and scope of the invention.

1-20. (canceled)
 21. An apparatus for forming an optical film,comprising: a casting section to cast a dope onto a support; a peelingsection peeling a film from the support; a tenter drying section dryingthe peeled film; and a winding section winding film in a roll over alength of at least 1000 m, wherein the film has film thickness of 20 to60 μm and variation in the film thickness within ±3.0 percent of thestandard film thickness.
 22. The apparatus of claim 21, wherein thesupport is a drum.
 23. The apparatus of claim 21, wherein the dopecomprises cellulose ester.
 24. A method for forming an optical film,comprising: casting a dope onto a support to form a film on the support;peeling the film from the support; drying the peeled film at a tenterdrying section; and winding the film in a roll over a length of at least1000 m, wherein the film has film thickness of 20 to 60 μm and variationin the film thickness within ±3.0 percent of the standard filmthickness.
 25. The method of claim 24, wherein the support is a drum.26. The method of claim 24, wherein the dope comprises a cellulose estercompound.
 27. The method of claim 24, wherein the cellulose estercompound is composed of ration of wood pulp cellulose/cotton lintercellulose=60/40 to 0/100 by weight.
 28. The method of claim 27, whereincellulose ester comprises a lower fatty acid esters of cellulose. 29.The method of claim 24, wherein the dope comprises benzotriazole. 30.The method of claim 24, wherein the dope comprises a plasticizers havinga solidifying point of no more than 20° C.
 31. The method of claim 27,wherein the cellulose ester has an acetated degree of 54 to 62 percent.32. The method of claim 31, wherein the cellulose ester has an acetateddegree of at least 59 percent.
 33. The method of claim 24, wherein thewinding speed is not less than 30 m/minutes.
 34. The method of claim 24,wherein the film is a polarizer protective film, a phase differencefilm, a reflective film, a visibility increasing film, a glareminimizing film, a non-reflective film, or an antistatic film.